In a topography simulation for a semiconductor device, it is required to calculate a flux of particles that reach a surface of a substance of the semiconductor device and contribute to the processing of topography of the substance in short time. The flux is called surface incident flux. For example, there is known a method of calculating the flux by using an aspect ratio of a concave portion formed in the substance. This method can calculate the flux in short time by approximating topography of the concave portion formed in the substance with one columnar shape, and analytically calculating the flux on a bottom surface of the concave portion.